This invention relates to electrical welders.
It has been proposed by the applicant to construct welding apparatus which includes an inverter with an output frequency in excess of the audio range of frequencies, and a transformer, the transformer having a center-tapped primary winding connected to the inverter and a secondary winding with output terminals for electrical connection to an electrode and work to be welded.
This welder forms the subject of the applicant's South African Pat. No. 81/2611.
During operation of a welder of this kind the primary winding has impressed across it a voltage which is twice the supply voltage. If the supply voltage in turn is derived from a rectified three-phase alternating voltage of, say, 380, volts, then the primary winding has a voltage across it, during operation, of approximately 1000 volts. Transistorized switching devices capable of handling this voltage at the operating current are difficult to obtain and expensive.
The patent literature discloses a number of welding circuit arrangements. The patent specifications known to the applicant are those of U.K. Pat. Nos. 2046537, 2019135, 1591185, 1570614, 1541068, 1530906, 1431379, 1420319, 1362163, 1308695 and 722494, U.S. Pat. Nos. 4,201,906, 4,159,409, 4,117,303, 4,048,468, 4,047,096, 4,038,515, 4,004,209, 3,973,165, 3,818,177, 3,728,516, 3,518,401, 3,304,485, 3,231,711, 3,211,953; and German Pat. No. 2325793.
A large number of these circuits suffer from the disadvantages described.
The specification of U.K. Pat. No. 2046537 discloses a different approach in which use is made of two controlled bridge type thyristor based inverters, each connected to a primary winding, which are operated in parallel. The inverters do not display the voltage doubling effect. However, since the inverters are controlled they function, or are switched, at a rate which is load independent. This can be harmful to the components of the circuit and if an overload occurs the thyristors can be burnt out unless additional precautionary measures are taken.